Conventionally, optical recording mediums such as CD-DAs, CD-ROMs, CD-Rs, CD-RWs, DVD-ROMs, DVD-Rs, DVD+/−RWs, and DVD-RAMs are widely used for viewing digital video contents and for recording digital data. The recording capacity required for such optical recording mediums is increasing every year. To meet this requirement, the so-called next generation optical discs that can store large video and data files are being commercialized. In such next generation optical discs, the increase in recording capacity is achieved by reducing the wavelength of the laser beam used for recording-reading to 405 nm. For example, in the Blu-ray Disc (BD) standard, which is one of the next generation DVD standards, the numerical aperture of an objective lens is set to 0.85, and this allows as much as 25 GB of data to be recorded in and read from a single recording layer.
To evaluate the quality of such optical recording mediums, reflection variations, for example, are used. The reflection variations are used to evaluate the variations of a signal obtained when a recording layer is read and are useful as a measure of the circumferential uniformity of an optical recording medium. In the (1,7) RLL modulation coding, the reflection variations are represented by the following equation.Reflection variations=(R8Hmax−R8Hmin)/R8Hmax 
Here, R8H corresponds to the reflectivity of an 8T space in an information recording layer in which the formation of recording marks results in a decrease in reflectivity. R8Hmax is determined based on the reflectivities of only 8T spaces in one revolution and means the highest one of the reflectivities. R8Hmin means the lowest one of the reflectivities of the 8T spaces in one revolution. The reason that the reflectivities of 8T spaces in one revolution vary in the range of R8Hmin to R8Hmax as described above is due to the circumferential variations of the characteristics of the material and shape of the optical recording medium (for example, variations in the thickness of the information recording layer, variations in the composition of the material, and variation of the thickness of a cover layer). Because of these influences, a bottom jitter value and an optimal recording power value vary in one revolution of the information recording layer. For example, the optimal recording power can vary in one revolution. In such a case, if a low optimal recording power is used to record information over one revolution, jitter in areas in which higher optimal recording powers should be used deteriorates considerably. When a high optimal recording power is used to record information over one revolution, jitter in areas in which lower optimal recording powers should be used deteriorates considerably. Therefore, an optimal recording power that simultaneously satisfies both the requirements is selected. However, the width of the margin for the optimal recording power decreases as the circumferential variations in optimal recording power increases. Due to the above reason, to evaluate reflection variations in an optical recording medium in advance is very important as a measure of uniformity of the optical recording medium.
Meanwhile, it is expected that the size of video and data files will increase more and more in the future. Therefore, it is contemplated to increase the capacities of optical recording mediums by using a multiple stack of information recording layers, as described in Non-Patent Documents (I. Ichimura et. al., Appl. Opt, 45, 1794-1803 (2006) and K. Mishima et. al., Proc. of SPIE, 6282, 628201 (2006)). For Blu-ray standard optical recording mediums, a technology for achieving an ultra large capacity (as much as 200 GB) by providing 6 to 8 information recording layers has been proposed.